Platform: we don't need to include windows.h here.

Did the same thing a while ago in Corrade/PluginManager/AbstractManager.h
so reusing it here.

src/Magnum/Platform/Implementation/OpenGLFunctionLoader.cpp

@@ -41,6 +41,10 @@
 #pragma warning(disable: 4055)
 #pragma warning(disable: 4054)
 #endif
+#ifndef WIN32_LEAN_AND_MEAN
+#define WIN32_LEAN_AND_MEAN
+#endif
+#include <windows.h>
 
 /* GLX-specific includes */
 #elif defined(CORRADE_TARGET_UNIX) && defined(MAGNUM_PLATFORM_USE_GLX)
@@ -93,9 +97,9 @@ auto OpenGLFunctionLoader::load(const char* const name) -> FunctionPointer {
      const PROC address = wglGetProcAddress(reinterpret_cast<LPCSTR>(name));
      const auto integerAddress = reinterpret_cast<std::ptrdiff_t>(address);
      if(address && integerAddress != 1 && integerAddress != 2 &&
-        integerAddress != 3 && integerAddress != -1) return address;
+        integerAddress != 3 && integerAddress != -1) return reinterpret_cast<FunctionPointer>(address);
 
-     return GetProcAddress(library, reinterpret_cast<LPCSTR>(name));
+     return reinterpret_cast<FunctionPointer>(GetProcAddress(library, reinterpret_cast<LPCSTR>(name)));
 }
 
 /* GLX-specific implementation */

src/Magnum/Platform/Implementation/OpenGLFunctionLoader.h

@@ -28,19 +28,16 @@
 #include "Magnum/Magnum.h"
 
 #if defined(CORRADE_TARGET_WINDOWS) && !defined(MAGNUM_PLATFORM_USE_EGL)
-#define WIN32_LEAN_AND_MEAN 1
-#include <windows.h>
+/* I didn't find a better way to circumvent the need for including windows.h */
+struct HINSTANCE__;
+typedef struct HINSTANCE__* HMODULE;
 #endif
 
 namespace Magnum { namespace Platform { namespace Implementation {
 
 class OpenGLFunctionLoader {
     public:
-        #if !defined(CORRADE_TARGET_WINDOWS) || defined(MAGNUM_PLATFORM_USE_EGL)
         using FunctionPointer = void(*)();
-        #else
-        using FunctionPointer = PROC;
-        #endif
 
         explicit OpenGLFunctionLoader();
         ~OpenGLFunctionLoader();